This invention relates to a counter-rotation twin-screw extruder for extracting a juice by squeezing vegetables and/or fruits. Extruders for such a purpose can generally be divided into a single-screw rotary type and a twin-screw rotary type. Twin-screw rotary extruders are mainly used for extracting edible oil from sesame seeds and may be further divided into co-rotation twin-screw extruders and counter-rotation twin-screw extruders. This invention relates more particularly to a counter-rotation twin-screw extruder used for extracting a juice from vegetables and/or fruits.
Examples of conventional twin-screw rotary extruder include co-rotation twin-screw extruders having screws mounted to two shafts adapted to rotate in the same direction as described in U.S. Pat. No. 3,450,034 and counter-rotation twin-screw extruders having screws mounted to two shafts adapted to rotate in opposite directions as described in Japanese Patent Publication Tokkai 2-251397. They are both provided with a plurality of tubular plates serving as an enclosure for a pair of mutually engaging parallel screws and a plurality of spacers disposed between these tubular plates such that gaps, or slits, are formed where there are no spacers between the tubular plates. When they are used for extracting edible oil, soybeans or oil-containing seeds such as sesame seeds are compressed between the parallel screws which are in mutually engaged relationship, and the oil thus extracted is collected through the slits formed between the plurality of tubular plates.
If these conventional twin-screw rotary extruders are directly used to extract a juice from vegetables and/or fruits, however, problems of the following kinds are encountered:
(1) For extracting edible oil from sesame seeds, a rotary extruder must be operated at a fairly high pressure because of the nature of the substance. Accordingly, the slits must be structured so as to withstand high pressures. In other words, the enclosure formed by tubular plates and spacers must be thick, and this makes the extruder excessively heavy. PA1 (2) If the thickness of the tubular plates is increased, the slits formed therebetween become deeper (in the radial direction) and the volume of the gaps correspondingly larger. Since unwanted voids are also created inevitably between the tubular plates and the spacers, the juice of vegetables and/fruits is easily contaminated with bacteria due to these gaps and voids. PA1 (3) Since the tubular plates and the spacers are affixed to the body of the extruder by way of through-bolts, it is difficult to disassemble the extruder for cleaning so as to prevent contamination as much as possible.